Firefox, the red-hot alternative web browser, is approaching its first big test as a mainstream product. One of Firefox’s best features has been its built-in pop-up blocker. Heck, sometimes it worked too well, preventing 2nd windows from opening when working in a web-based email program. But I was happy it was on the case.

Today, I noticed (as did the folks at Adrants) that a couple of sites snuck in pop-under ads, something that Firefox had been quite good at preventing previously. That’s the price you pay for popularity: increased attention from sleazy folks.

So how long until Firefox responds and patches this up? (And then how long before the cat-and-mouse game speeds up and they’re working on patches as much as what they were trying to supplant?) It better be quick or that rising tide of Firefox evangelists may settle into being yet another group of carping Web addicts.